Don’t miss Mercury’s best evening show of the year at dusk from 20 February

By Ade Ashford

Innermost planet Mercury’s best evening showing of the year for northern observers occurs during the fortnight starting 20 February, as simulated in this looping animation. UK skywatchers should find a location that offers an unobstructed view of the horizon from west-southwest through west at the end of civil twilight, which is about 40 minutes after sunset for the heart of the British Isles. Mercury is brightest at the start of the period, and best placed from 25 February through 2 March when it’s 10° high (about the span of a fist at arm’s length) at the end of civil twilight. Caution: only look for Mercury after sunset. AN graphic by Ade Ashford.Every keen stargazer feels a quickening of the pulse at seeing a clear sky at dusk. This feeling intensifies when one is in with a chance of getting a naked-eye glimpse of Mercury. It is rightly regarded as somewhat elusive, but the innermost planet’s not particularly difficult to track down if you’re prepared to do a little homework to find out when and where to look for it. Mercury attains an elongation of 18.1 degrees east of the Sun at 1:25am GMT on 27 February.

This year’s finest evening apparition of Mercury for observers in the British Isles lasts about a fortnight starting 20 February. First, you need to find a location that offers you an unobstructed view of the horizon from west-southwest through west at the end of civil twilight, which is about 40 minutes after sunset for the heart of the UK. For any given day you can find out the time when civil twilight ends for where you live from our interactive online Almanac.

Mercury is brightest (magnitude -1.0) at the start of this two-week-long observing window around 20 February, and best placed from 25 February through 2 March when it’s 10° high at the end of civil twilight as seen from the centre of the British Isles. And I make no apologies for repeating the usual warning to protect your eyesight: never look for Mercury with binoculars or telescopes until after sunset. Happy hunting!

Why this is a good time to look for Mercury
As winter segues into spring, many casual skywatchers at temperate northern latitudes such as the UK express surprise at how rapidly the waxing young crescent Moon ascends in the western sky on successive nights, in stark contrast to dusk in autumn when the Moon appears to hug the southwestern horizon. The illustration below explains why this is so.How the season makes a huge difference to the visibility of a young Moon or Mercury at sunset. The diagram represents a lunar crescent and the innermost planet at its greatest easterly elongation from the Sun. As seen from the UK during the spring, more of the Moon and Mercury’s elongation from the Sun translates into height above the horizon because the angle of the ecliptic is steepest to the western horizon, hence the Moon or Mercury is visible for longer. The converse is true in the autumn when the ecliptic angle is shallow to the western horizon. Far from this idealised scenario, the Moon and planets can also appear north or south of the ecliptic, further affecting their visibility. AN graphic by Ade Ashford.

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With the transit of Mercury just two days away, interest in this comparatively rare event is growing fast. Given the favourable timing of this 7½-hour phenomenon for the UK, many will be able to view it at lunchtime or after work. If you don’t have suitably equipped telescope, join one of the many transit-viewing activities hosted by astronomical organisations nationwide.

Now that planet Saturn is effectively lost in the dusk twilight for UK-based observers, you may be wondering what has happened to the other four bright naked-eye planets. Far from disappearing, they have just transferred to the morning sky. From 8—11 October, the waning crescent Moon acts as a guide to Venus, Mars, Jupiter then Mercury in the eastern dawn sky.

Observers in the UK and Western Europe should find an observing location offering an unobscured eastern horizon an hour before sunrise on Sunday, 10 September to see innermost planet Mercury just 0.6 degrees from Regulus, the brightest star in Leo. Conspicuous planet Venus is your convenient celestial guide to finding Regulus, Mercury and Mars.

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